MHD_response_execute_upgrade_(): deduplicate code.
MHD_response_execute_upgrade_(): deduplicate code.

ghaderer/libmicrohttpd,ghaderer/libmicrohttpd,maru/libmicrohttpd-http2,maru/libmicrohttpd-http2,maru/libmicrohttpd-http2,ghaderer/libmicrohttpd